- BOULDER--Carbon dioxide emissions over the next century could increase
wintertime precipitation in the U.S. Southwest and Great Plains by 40%
as global average temperature rises 3 degrees Fahrenheit (2 degrees Celsius),
according to latest results from a new climate system model developed at
the National Center for Atmospheric Research by NCAR, university, and other
laboratory scientists. Reducing the buildup of carbon dioxide concentrations
over the next century by one half largely dries up the extra rain and snow
and slows the global temperature rise to 2 degrees F (1.5 degrees C). The
model results were announced this week in Atlanta. The study was funded
in part by the National Science Foundation (NSF), NCAR's primary sponsor.
- The NCAR model simulated the earth's
climate from 1870 to 1990 and then continued the simulation to 2100 under
two different scenarios. The first was a "business-as-usual"
increase in greenhouse gases in which atmospheric carbon dioxide doubles
over the next century. In the second, carbon dioxide increases are stabilized
at 50% above today's concentrations. In the first projection, changes in
precipitation vary markedly by region and by season. Within the United
States, the greatest increases occur in the Southwest and Great Plains
in winter and substantially exceed the range of natural variability. Precipitation
changes are reduced when carbon dioxide emissions are limited, according
to the model.
- Global average temperature climbs by
3 degrees F (2 degrees C) for business as usual and 2 degrees F (1.5 degrees
C) when carbon dioxide emissions are limited. These changes are three to
four times larger than the warming that has occurred since 1900. On the
continental scale, carbon dioxide stabilization reduces climate warming
over Eurasia more than over North America.
- NCAR scientist Tom Wigley says, "These
results show that we will experience not only future climate change, but
also the results of policies to reduce these changes, in ways that are
not simply related to changes in the global mean temperature. Policy decisions
about reducing greenhouse emissions should not, therefore, be dictated
by projected changes in global mean temperature alone."
- The model shows no clear separation between
the business-as-usual and the stabilization cases until around 2060, even
though the carbon dioxide concentrations begin to diverge in 2010. The
half-century lag until the changes in greenhouse emissions begin to affect
the climate noticeably is the result of large thermal inertia in the earth's
climate system, especially in the oceans, say the scientists.
- The NCAR model's special features help
push the science of climate modeling into new territory. It is one of the
world's first global models not to require special corrections to keep
the simulated climate from drifting to an unrealistic state. It is also
one of only a handful of models in the world capable of realistically simulating
the chemistry and transport of individual greenhouse gases and sulfur compounds.
The model employs a more realistic scenario for future emissions of sulfur
dioxide, a form of industrial pollution that cools the climate. Assuming
that societies will take steps to reduce sulfur dioxide emissions over
the next century, the scientists incorporated this decline into the model.
The sulfur dioxide cooling effect gradually diminishes, allowing the simultaneous
greenhouse warming to emerge more clearly.
- Data from these NCAR climate system model
runs are available to the scientific community for research into possible
effects of climate change on human health, water resources, agriculture,
natural ecosystems, and the economy. Scientists from both NCAR and the
National Oceanic and Atmospheric Administration worked on the study, which
was funded by NSF and ACACIA (A Consortium for the Application of Climate
Impact Assessments). ACACIA is a joint NCAR-industry program sponsored
by electric utility research organizations in the United States, Japan,
and The Netherlands, and by NCAR and NSF. The simulations were run on supercomputers
at NCAR and in Japan.